1. Field of the Invention
The present invention relates to a high-frequency signal generator for use in a millimeter wave FM radar module installed on a motor vehicle.
2. Description of the Prior Art
Radar devices for use on motor vehicles such as automobiles in combination with warning units for preventing collisions are required to have a high degree of resolution for detecting objects in close distances of about several tens of centimeters. In view of such a high-resolution requirement, an FM radar is preferable to a pulse radar for use in the vehicle-mounted radar devices. Since the maximum range that may be detected up to a target such as a preceding motor vehicle or an upcoming motor vehicle is of a relatively short distance of about several hundred meters, it is suitable for such a radar to use radiowaves in the millimeter range which have a frequency of about 60 GHz and can be attenuated greatly upon propagation in order to prevent radiated radiowaves from being propagated beyond a necessary range and also from interfering with existing microwave communications equipment. Use of millimeter waves is also preferable from the viewpoint of reducing the size of a radar module including an antenna, FM signal generators in front and rear stages, a mixer, and other components.
Heretofore, FM radar modules in the millimeter range are constructed in the form of a microstrip line or a waveguide. Because the microstrip line radiates a large amount of power, it suffers a large loss and tends to cause interference between a plurality of modules, resulting in a reduction in measuring accuracy. The waveguide is disadvantageous in that its circuit is large in size and expensive.
One of the attempts to solve the above problems is a non-radiative dielectric (NRD) waveguide as disclosed in an article "Millimeter wave integrated circuit using a non-radiative dielectric waveguide" written by Yoneyama et al. and published in the Journal of Electronic Information Communications Society, Vol. J 73 c-1 No. 3 pp. 87-94, March 1990. The disclosed non-radiative dielectric waveguide comprises two confronting conductive plates spaced from each other by a distance slightly smaller than a half wavelength and a rod-shaped dielectric member inserted between the conductive plates for allowing only propagations along the rod-shaped dielectric member. The upper and lower surfaces of the non-radiative dielectric waveguide are completely shielded by the conductive plates. Since the distance between the conductive plates is shorter than the half wavelength, radiowaves are fully prevented from leaking laterally out of the non-radiative dielectric waveguide. Therefore, any power radiation from the non-radiative dielectric waveguide is very small, effectively avoiding radiation loss in a module and interference between modules.
Various components including a directional coupler and an isolator can easily be fabricated by positioning non-radiative dielectric waveguides closely to each other or adding ferrite. Therefore, modules employing non-radiative dielectric waveguides can be made smaller than the conventional microstrip arrangement where components are separately produced and interconnected by a waveguide. The above article also discloses small-size, high-performance transmitter and receiver structures for use in the millimeter wave band which employ non-radiative dielectric waveguides.
The article also discloses a gunn oscillator as shown in FIG. 11 of the accompanying drawings and a non-radiative dielectric waveguide for guiding signals which are generated in the millimeter wave band by the gunn oscillator to an antenna or the like. As shown in FIG. 11, the gunn oscillator and its surrounding circuits comprise upper and lower conductive plates 31, 32 serving as a non-radiative dielectric waveguide, a diode mount 33 sandwiched between the upper and lower conductive plates 31, 32, a gunn diode 34 threaded in the diode mount 33, a printed-circuit board 35 fixed to a side of the diode mount 33, a dielectric rod 40 for guiding a signal generated in the millimeter wave band by the gunn diode 34 to an antenna or the like (not shown), and a metal foil oscillator 41 for guiding the signal generated in the millimeter wave band by the gunn diode 34 to the dielectric rod 40.
In FIG. 11, the distance between the upper and lower conductive plates 31, 32 is set to a value slightly smaller than half the wavelength of the signals used in the millimeter wave band. If the signals have a frequency of about 60 GHz, for example, then the distance between the upper and lower conductive plates 31, 32, and hence the thickness of the diode mount 33 is of a small value of about 2.5 mm. Commercially available packaged gunn diodes are mounted on a heat-radiating stud which is of a diameter ranging from 3 to 4 mm. Therefore, it is necessary to machine them to make them ready for use in actual applications, as shown in FIGS. 12(A) and 12(B). First, as shown in FIG. 12(A), a gunn diode 35 is threaded in a metal block which is 5 to 6 mm thick. Then, as shown in FIG. 12(B), upper and lower portions of the metal block are cut off to reduce the thickness thereof to about 2.5 mm, and grooves dimensioned to a 1/4 wavelength are defined in the metal block to prevent the signals from leaking out. In this manner, the gunn diode 34 is mounted on the diode mount 33.
In a high-frequency signal generator which employs the gunn diode disclosed in the above article, the diode mount is sandwiched between the upper and lower conductive plates so as to be positioned with respect to the metal foil resonator. Therefore, the electric characteristics including the oscillation frequency and the output level tend to be fluctuate to a large extent due to a small positional shift between the diode mount and the dielectric rod, particularly a small rotation in the directions indicated by the arrows in FIG. 11.
Inasmuch as the gunn oscillator requires complex machining as shown in FIGS. 12(A) and 12(B) to fabricate the diode mount 33 that is of a small thickness, the process of manufacturing the gunn oscillator is time-consuming, and the produced gunn oscillator is costly.
The metal foil resonator is dimensionally adjustable for adjusting the oscillation frequency of the high-frequency signal generator. However, the process of adjusting the dimensions of the metal foil resonator is relatively complex to carry out.
In the case where the high-frequency signal generator is used as an FM signal generator in an FM radar module on a motor vehicle, the reverse side of a dielectric plate is bonded to an end face of a dielectric rod by an adhesive or the like, thus fixing the metal foil resonator to the dielectric rod. When the adhesive layer is broken due to vibrations or shocks applied while the motor vehicle is running, the metal foil resonator may peel off the end face of the dielectric rod.
While the above article shows the high-frequency signal generator using the gunn diode, it does not discuss any optimum arrangement for an FM signal generator for use in an FM radar module.